[petsc-users] IJacobian "PetscReal a"

[Jin, Shuangshuang]

Hello, when I look at IJacobian function in the example: 
http://www.mcs.anl.gov/petsc/petsc-current/src/ts/examples/tutorials/ex19.c

static PetscErrorCode IJacobian(TS ts,PetscReal t,Vec X,Vec Xdot,PetscReal 
a,Mat *A,Mat *B,MatStructure *flag,void *ctx)
{
  PetscErrorCode ierr;
  PetscInt       rowcol[] = {0,1};
  PetscScalar    *x,J[2][2];

  PetscFunctionBeginUser;
  ierr    = VecGetArray(X,&x);CHKERRQ(ierr);
  J[0][0] = a;    J[0][1] = -1.;
  J[1][0] = 1.;   J[1][1] = -1. + x[1]*x[1];
  ierr    = 
MatSetValues(*B,2,rowcol,2,rowcol,&J[0][0],INSERT_VALUES);CHKERRQ(ierr);
  ierr    = VecRestoreArray(X,&x);CHKERRQ(ierr);

  ierr = MatAssemblyBegin(*A,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
  ierr = MatAssemblyEnd(*A,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
  if (*A != *B) {
    ierr = MatAssemblyBegin(*B,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
    ierr = MatAssemblyEnd(*B,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
  }
  *flag = SAME_NONZERO_PATTERN;
  PetscFunctionReturn(0);
}

There's a PetscReal a. Is it a constant? And if the value of a known outside of 
the IJacobian function, saying the main() part.

If it is, then as shown in this example, J[0][0] = a; J[0][1] = -1.; J[1][0] = 
1.; are constant through all the iterations. How can I use 
MatRetrieveValues(*A) and MatStoreValues(*A) to reuse them?

I'm asking this question because I have a large Jacobian matrix, but half of 
the matrix contains constant values. However, I don't know what's "PetscReal a" 
when I try to use it to compute my constant elements of Jacobian matrix which 
doesn't depend on the value of x at all.

Thanks,
Shuangshuang